Exotic Alloy Fastener Glossary

Technical definitions for exotic alloy fastener terminology. Each term includes specific data points, applicable standards, and relevance to fastener material selection.

C G H I P S

Creep Strength

Creep strength is the stress producing a specified deformation rate at a given temperature over time. At elevated temperatures, metals permanently deform under constant stress even below yield strength. Expressed as stress for a defined creep rate (e.g., 0.01% per 1000 hours) or total strain (e.g., 1% in 100,000 hours / ~11.4 years). Above 540°C for steel and 650°C for nickel alloys, creep strength rather than tensile/yield strength becomes the controlling design parameter for fasteners per ASME BPVC Section II Part D.

Inconel 625

Crevice Corrosion

Crevice corrosion occurs in confined spaces where electrolyte stagnates and becomes oxygen-depleted. In fastener assemblies, common locations include under bolt heads, between nut and flange faces, and in threaded regions. The oxygen-depleted crevice becomes anodic, creating a concentration cell with accelerated dissolution. The pH within the crevice drops (acidifies), further accelerating attack. Crevice corrosion is more aggressive than pitting and initiates at lower temperatures. Alloys with high PREN values resist it better; Super Duplex 2507 (PREN ≥42.5) outperforms standard grades.

Duplex 1.4462 Hastelloy C-276

Galvanic Corrosion

Galvanic corrosion is accelerated attack on a less noble (anodic) metal when electrically coupled to a more noble (cathodic) metal in an electrolyte. In fastener applications, this occurs when fasteners of one alloy join components of a dissimilar alloy. The design rule: the fastener should always be the more noble (cathodic) material, so the larger surface area of joined components distributes the corrosion current over a larger area, reducing the corrosion rate. Example: use Inconel 625 or titanium fasteners to join steel structures, never carbon steel bolts in stainless flanges.

Titanium Grade 2 Inconel 625

Hot Working Temperature Range

The temperature range for plastic deformation (forging, rolling, extruding) without cracking or unfavorable microstructures. Ranges are alloy-specific: Inconel 625 at 925–1175°C, Inconel 718 at 955–1120°C, Hastelloy C-276 at 870–1230°C. Working below minimum risks cracking from insufficient ductility. Working above maximum risks incipient melting, excessive grain growth, or unfavorable phase precipitation that degrades mechanical properties.

Inconel 625 Hastelloy C-276

Intergranular Corrosion (IGC)

Intergranular corrosion is localized attack along grain boundaries caused by compositional differences between the boundary region and grain interior. Commonly caused by sensitization where chromium carbide (Cr23C6) precipitation creates chromium-depleted zones below the 12% Cr threshold needed for passivation. In nickel alloys and stainless steels, these depleted zones become anodic, creating galvanic micro-cells. Prevention methods: use low-carbon 'L' grades (e.g., 316L with <0.03% C), stabilized grades with Ti or Nb, or solution anneal after thermal exposure.

Inconel 625 Hastelloy C-276

Passivation

Passivation is the formation of a thin (1–3 nm), protective, chromium-rich oxide film on stainless steels and nickel alloys that provides corrosion resistance. This film forms spontaneously in oxidizing environments. In fastener manufacturing, passivation is also a chemical treatment per ASTM A967 using nitric or citric acid to remove free iron and contaminants, promoting a uniform passive layer. Localized plastic deformation under stress can rupture the film, exposing bare metal — a key mechanism in SCC and pitting initiation.

Inconel 625 Hastelloy C-276 Duplex 1.4462

Pitting Resistance Equivalent Number (PREN)

PREN is a numerical index comparing pitting and crevice corrosion resistance based on chemical composition: PREN = %Cr + 3.3 x %Mo + 16 x %N. Higher values indicate better resistance. Key thresholds: PREN > 32 is suitable for seawater service; PREN > 40 earns the 'super' designation (e.g., Super Duplex 2507). Reference values: 316L stainless steel PREN ~24, Duplex 2205 PREN ~35, Super Duplex 2507 PREN ~43. PREN is defined in EN ISO 3506-6 for fasteners with PREN ≥ 40.

Duplex 1.4462

Precipitation Hardening (Age Hardening)

Precipitation hardening increases strength by forming fine precipitate particles that impede dislocation movement. The process: (1) solution anneal to dissolve elements, (2) rapid cool to create supersaturated solution, (3) age at 425–900°C to precipitate strengthening phases. In Inconel 718, the primary phases are gamma prime (Ni3Al) and gamma double prime (Ni3Nb), enabling 1379 MPa tensile strength vs. 827 MPa for solid-solution Inconel 625. Per ASTM B637, 718 bar stock must meet specific aging parameters.

Inconel 625

Proof Load

Proof load is the maximum tensile force a fastener can withstand without permanent (plastic) deformation, remaining entirely in its elastic region. Typically 85–95% of yield strength. Testing involves applying the specified force for 10 seconds, then measuring length — any permanent elongation indicates failure. Proof load is the design basis for fastener preload, with recommended assembly preload at 65–75% of proof load. Defined in ASTM F468 for nonferrous fasteners and EN ISO 3506 for stainless steel fasteners.

Sensitization

Sensitization is the precipitation of chromium carbides (Cr23C6) at grain boundaries when stainless steels or nickel alloys are held at 425–850°C during welding, heat treatment, or service. The carbides absorb chromium from the surrounding matrix, creating depleted zones below 12% Cr that lose passive film protection and become susceptible to intergranular corrosion. Prevention: use low-carbon grades ('L' grades with <0.03% C, e.g., 316L), stabilized grades containing Ti or Nb, or solution anneal after exposure.

Inconel 625 Hastelloy C-276

Solution Annealing

Solution annealing heats the alloy to 1000–1200°C (for nickel alloys) to dissolve all elements and precipitates into a single-phase solid solution, followed by rapid cooling to retain elements in supersaturated solution. For solid-solution-strengthened alloys like Inconel 625 and Hastelloy C-276, this is the final heat treatment. For precipitation-hardenable alloys like Inconel 718, solution annealing is the first step before aging at 425–900°C to form strengthening precipitates.

Inconel 625 Hastelloy C-276

Stress Corrosion Cracking (SCC)

Stress corrosion cracking is the formation and growth of cracks under simultaneous tensile stress and a specific corrosive environment. SCC requires three conditions: a susceptible material, a specific corrosive medium, and sustained tensile stress. Cracks propagate intergranularly or transgranularly, causing sudden brittle failure well below the nominal tensile strength. In fastener applications, SCC is critical for austenitic stainless steels in chloride environments (above 60°C for 316L) and for carbon steels in H2S service per NACE MR0175/ISO 15156. Alloys like Inconel 625 and Hastelloy C-276 are immune to chloride SCC.

Inconel 625 Hastelloy C-276 Duplex 1.4462